CDC warns of Salmonella risk from tomatoes

Sep 6, 2007 (CIDRAP News)  Consumers should take precautions to limit their risk of contracting Salmonella infections from raw tomatoes, which may have sickened more than 79,000 people in a dozen outbreaks since 1990, the Centers for Disease Control and Prevention (CDC) said today.

In an analysis of four recent multistate Salmonella outbreaks linked to tomatoes served in restaurants, the CDC advises, among other things, that cut tomatoes should be refrigerated within 2 hours. The analysis was published in the Sep 7 issue of Morbidity and Mortality Weekly Report.

The article says that at least 12 multistate Salmonella outbreaks linked to tomatoes have been reported to the CDC since 1990, causing about 1,990 confirmed illness cases. Because more than 97% of Salmonella infections are not confirmed by culture, "these outbreaks might have resulted in as many as 79,600 illnesses," the CDC states.

The report describes four salmonellosis outbreaks associated with restaurant tomatoes that involved 459 culture-confirmed cases in 21 states in 2005 and 2006. Most of the affected states were in the East and Midwest. None of the cases were fatal.

From July to November 2005, 72 confirmed cases associated with the same strain of Salmonella Newport (matched by pulsed-field gel electrophoresis) were identified in 16 states. Eight patients were hospitalized. A case-control study indicated that illness was associated with eating raw, large, round tomatoes at restaurants, but no single restaurant or chain was implicated. Investigators concluded that the contaminated tomatoes had been grown at two farms on the eastern shore of Virginia. The outbreak strain was found in irrigation pond water near tomato fields in the region.

In the second outbreak, 82 cases featuring the same strain of Salmonella Braenderup were identified in eight states in November and December 2005. Eighteen patients were hospitalized. A case-control study pointed to a particular chain restaurant where patients had eaten raw, diced Roma tomatoes, the report says. The tomatoes had been grown in one of two fields in Florida and were diced and packaged at a firm in Kentucky before being shipped to the restaurant. Investigators found Salmonella in environmental samples from the Florida farm, but it was not the outbreak strain.

A third outbreak involved 115 cases confirmed between July and November of 2006 in 19 states. The cases, which caused eight hospitalizations, featured the same strain of Salmonella Newport as the 2005 outbreak. Again, a case-control study pointed to raw tomatoes eaten in restaurants as the likely cause, but no single restaurant or chain was implicated. The source of the tomatoes was not determined.

The fourth outbreak featured 190 confirmed cases from 21 states in September and October of 2006, all involving the same strain of Salmonella Typhimurium, with 24 patients hospitalized. As evidenced by a case-control study, cases were linked with eating large, round tomatoes at a restaurant.

The implicated tomatoes were traced to one packing house in Ohio that handled tomatoes from 25 fields. The production season had ended by the time the facility was implicated, so the Food and Drug Administration (FDA) waited until this year to investigate the situation. The investigation was completed in August, but the results were not included in the CDC report.

The CDC says the widely dispersed nature of the four outbreaks suggests that contamination occurred early in the distribution chain, such as at the farm or packing facility, rather than in restaurants. "These recurrent multistate outbreaks indicate that the tomato-growing environment is an ongoing source of contamination of tomatoes," the report says.

Exactly how tomatoes get contaminated is not known, but experiments have pointed to several possibilities, the report states. For example, tomatoes can absorb Salmonella when they are immersed in water that's cooler than the tomatoes, according to one study. Another study indicated that tomatoes can become internally contaminated when stems and flowers of the plants are inoculated with Salmonella from contaminated water.

Further, cutting can transfer contamination from the skin of a tomato to the interior, and cut tomatoes provide a good medium for bacterial growth, the CDC says.